For many years produce has been cooled by simply storing it in a refrigerated room, a process known as room cooling. This method is generally sufficient for keeping produce at a low temperature once it has been cooled, but it often does not remove field heat rapidly enough to maintain the quality of highly perishable crops. Room cooling is very often inadequate for produce stored in large containers, such as bulk boxes or pallet loads, and for produce that requires immediate cooling.
In the room cooling process, heat is removed slowly from only that produce near the outside of the container. Near the center of a container, heat is often generated by natural respiration more rapidly than it can be removed, causing the temperature to rise. Some types of produce, such as strawberries, must be cooled as quickly as possible after harvesting to preserve its fresh quality. Even a delay of several hours may be enough to reduce quality considerably. In such cases, room cooling is not fast enough to prevent serious damage.
To preserve quality, fresh produce should be cooled to its lowest safe (optimum) storage temperature as quickly as is practical and economical. Forced-air cooling is much faster than room cooling and is being used increasingly in North Carolina to cool produce quickly. It offers these advantages:
On the other hand, forced-air cooling is likely to increase the overall energy cost slightly by increasing electrical demand, a measure of the rate at which electricity is consumed. Demand cost contributes significantly to the electrical power bill for most cooling facilities. To reduce demand cost, produce should not be cooled any faster than necessary. Forced-air cooling can also increase cooling cost by increasing the cooling load per unit of time. Quicker cooling requires larger refrigeration units, the cost of which must be amortized over the life of the facility. The benefits of forced-air cooling, however, far outweigh the costs.
Forced-air cooling is a useful tool for preserving the quality of fresh produce. It is most effective when the produce demands quick cooling or when the amount of produce to be cooled per day or week is large enough to justify the increased equipment and electrical demand costs.
The graph of time and temperature in Figure 1 illustrates the response of a typical commodity to airflow rates. The beginning temperature of the produce (pulp temperature) is represented by Ta. This temperature varies with ambient conditions and the amount of field heat in the produce; it normally ranges from 60 to 90 F.
Figure 1. Cooling time for various airflow rates.
The desirable air temperature inside the cooling room, Tb, depends primarily on the type of commodity. Few types of produce will tolerate temperatures below freezing, although some, such as strawberries and apples, require near-freezing storage temperatures. Many others, such as squash and cucumbers, will sustain chill injury if exposed to temperatures lower than 45 F.
The interior air temperature, Tb, is best measured by a thermometer positioned away from the exterior walls, ceiling, and produce containers. In practice, this temperature is regulated by the thermostat setting of the refrigeration system. Correct storage temperatures for most types of produce grown in North Carolina are given in Extension Publication AG-414-1, Maintaining the Quality of North Carolina Fresh Produce: Introduction to Proper Postharvest Cooling and Handling Methods.
Curve A in Figure 1 represents the comparatively slow rate of cooling that can be expected without forced air movement (room cooling). Curves B and C demonstrate the increase in cooling rate possible with airflow rates of 1/2 and 1 cubic foot per minute per pound of produce, respectively.
The rate of cooling, represented by the slope of the curve, decreases as the produce temperature approaches the temperature of the room air. Reducing the temperature the last few degrees may take from several days to several weeks and is of little practical importance. In comparing cooling times for various methods, the time required to lower the pulp temperature to 7/8 of the difference between Ta and Tb is the value often used. On the graph, the 7/8 cooling time in still air is more than 7, compared to just over 1 for produce cooled with an airflow of 1 cubic foot per minute per pound of produce.
Since the cooling rate for a forced-air system is much greater than for room cooling, a refrigeration system of larger capacity may be required. Whether an existing system sized for room cooling will be sufficient for conversion to forced-air applications depends on a number of factors. These include the size of the original system, the anticipated future cooling loads, and the use factor of the facility. A qualified refrigeration contractor or a refrigeration specialist with the Agricultural Extension Service can help in determining whether additional refrigeration capacity will be needed.
Several different fan positions and produce stack-ing arrangements have proven successful for forced-air cooling. The shell arrangement shown in Figure 2 uses a portable pallet-mounted fan and is preferred by many because of its versatility. Two parallel rows of produce, positioned approximately 2 to 3 feet apart and covered by a cloth or plastic strip, form the shell. Cold air pulled through the produce flows through the space between the rows and out through the fan. In another arrangement known as the "cooling wall" (Figure 3), the fans are located permanently along one wall. This design might be more convenient for producers and shippers who handle large volumes of produce, especially if they always handle the same commodity or compatible ones. Both types of systems are widely used in North Carolina.
Figure 2. Shell arrangement with portable pallet-mounted fan.
Figure 3. Cooling wall arrangement with permannently mounted fans.
Because air is forced through the produce packages by the difference in air pressure between the opposite sides, it is necessary to fill the containers properly and stack them in such a way as to minimize voids and openings. Openings between containers allow the air to circumvent the produce, reducing cooling efficiency. Baffles may be positioned over unavoidable openings to direct the air through the produce. Double stacking should be avoided since even powerful forced-air fans have difficulty pulling air through more than one pallet width (3 to 4 feet) of produce.
In addition to controlling the temperature and airflow, it may be necessary to control the humidity. Moving air tends to remove water from the surface of produce, causing wilting, shrinkage, and general loss of quality and value. Most produce items require a relative humidity in the range from 90 to 98 percent if they are to be kept for more than a few hours in cold storage before shipment.
If the condensation from the evaporator coils is drained to the outside, the humidity in refrigerated rooms may become quite low. The amount of condensation that collects on these coils may be decreased substantially by limiting the temperature drop through them to 5 F. This can be accomplished by increasing the size or number of the coils. In practice, relative humidity levels above 80 or 85 percent cannot easily be achieved without some type of humidification system or very careful management.
Low humidity may be corrected by various types of commercial humidification systems. Many operators simply hose down the floors from time to time, but this approach may not be consistent with good sanitation nor particularly effective in many situations. On the other hand, excessively high humidity for long periods can also be detrimental because it encourages the growth of molds and fungi. Although a high-quality humidistat can be used to control the humidifier, the most consistently accurate method to measure relative humidity is with a wet-bulb thermometer. Construction details and directions for the proper operation of a wet-bulb thermometer may be obtained from your county Agricultural Extension Service agent.
Figure 4. Typical fan curve.
In addition to airflow rate and temperatures, several other variables influence the time required to cool produce with forced air, including the size and shape of the produce and the configuration and venting of the containers. In practice, however, an airflow rate of approximately 1 to 3 cubic feet per minute at 1/2 inch static pressure should be sufficient for most applications. Accurate cooling data for a specific set of conditions can be acquired only by conducting field tests.
The airflow rate through a fan system in a cooling setup may be measured with sufficient accuracy using an inexpensive U-tube manometer mounted to the fan (Figures 5 and 6). U-tube manometers are designed to measure differences in air pressure. One side of the manometer is connected on the upstream side of the fan as far as possible from the blades and the other end is open to the room air. By knowing the static air pressure through the fan and consulting the performance chart usually supplied with new fans, the airflow rate may be accurately determined.
Figure 5. U-tube manometer.
Figure 6. Pallet-mounted forced-air cooling fan.
In addition, it is also useful to mount a thermometer on the downstream (exhaust) side of the fan. By comparing the temperature of the air as it exits the fan with the temperature of the room air, it is possible to determine the cooling rate. The following example will illustrate the procedure.
A manometer attached to a fan pulling air through 8,000 pounds of peppers shows a static pressure difference of 1/2 inch of water. The performance chart for this fan shows an airflow rate of 14,000 cubic feet per minute at this static pressure. The room air temperature is measured at 45 F and the temperature of the air exiting the fan is 52 F. Since raising the temperature of 54 cubic feet of air 1 F requires one Btu of heat energy:The rate of heat loss changes continuously during the cooling period. As shown in Figure 1, it is greatest at the beginning of the cooling cycle (when the difference between product temperature and air temperature is greatest) and ultimately tapers off to zero.(14,000)(52-45) Heat Loss = --------------- = 1,815 Btu/minute 54It takes approximately 1 Btu to lower the temperature of 1 pound of peppers 1 F. Therefore, at a heat loss rate of 1,815 Btu per minute, the temperature of the 8,000 pounds of peppers is being reduced approximately 1 F every 4.4 minutes.
8,000 ----- = 4.4 1,815
The interior of a cooling room is often damp or even wet. Fan motors should therefore be of the totally enclosed, fan-cooled type (TEFC) and fully grounded according to local electrical codes to prevent shock.
It is also a good idea to control the fan with a line-voltage thermostat mounted in the airstream. The thermostat will stop the fan when the produce has cooled to a predetermined point, thus saving energy. It will also reduce the drying effects of the cooling air because it will not allow the fan keep running for an extended period after the produce has cooled. The thermostat should be set at 5 to 8 F above the temperature of the room air.
Plans and directions for building a portable pallet-mounted fan similar to the one shown in Figure 6 are found at the end of this publication. This fan is capable of moving more than 11,000 cubic feet of air per minute against a static pressure of 3/8 inch of water. It is suitable for use in a variety of forced-air cooling applications.
M. D. Boyette, Extension Agricultural Engineering Specialist
L. G. Wilson, Extension Horticulture Specialist
E. A. Estes, Extension Marketing Specialist
Sponsored by the Energy Division, North Carolina Department of Commerce, with petroleum violation escrow funds, in cooperation with the Agricultural Extension Service, North Carolina State University. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not reflect the views of the Energy Division, North Carolina Department of Commerce.
THE NORTH CAROLINA AGRICULTURAL EXTENSION SERVICE